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Leak Testing Basic Principles - Background Information of Leaks and Leak Testing From TQC
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5/31/13 Leak Testing Basic Principles - Background Information of Leaks and Leak Testing from TQC
www.tqc.co.uk/leaktesting/leak_basic_principles.htm 1/3
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leak testing - basic principles
What is a Leak?
A leak is a flow of gas (or liquid) through the wall of a vessel (via an imperfection
such as a hole, crack or bad seal).
Leaks require a pressure difference to generate the flow; they always go from
higher pressure to lower pressure.
Leaks are pictured as going from positive pressure (inside an object) to outside (atatmospheric pressure).
This is not always the case (a leak could be from atmosphere to inside anevacuated object), but it helps to think about it this way because the units and
terminology are based on this model.
Leak Testing - what units do you use?
For leaks of air into atmosphere, units are usually expressed as mm3 or cm3 (cc)
per second or minute. So 16.6 mm3/sec = 1 cm3/min. A bubble under water is
about 30 - 50 mm3, so 1 bubble per second is about 30 mm3/sec or 2 cm3/min. A
standard unit of leakage which takes account of air pressure is the mbarl/sec.(Millibar-litre per second). A leak into atmosphere of 1 mbarl/sec is equivalent to
a volume leak of 1000 mm3/sec.
Leak Testing - what technique should be used?
Key questions at the start of any leak test requirement are:-
What size is the component and what isit’s internal volume?
Is there access to inside or is it a sealedunit?
What is the leak limit? Is it rigid or flexible?
Does it have hidden internal volumes
that may affect leak measurements?Are parts at ambient temperature?
Are the parts clean and dry?What is the surface finish of any sealing
surfaces?
Based on the acceptable leak rate limit (shown in ml/sec) alone the following test
method can be used
Tracer Gas Flow Rate
Water Submersal / Dunk Tank
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Leak Testing - generic systems
pressure/vacuum
The test piece and the reference volume
are simultaneously pressurised (or
evacuated) to a preset pressure. The air
in the system is then allowed to stabilise,with the supply valves all closed. The
Differential Pressure Transducer is
automatically zeroed.
After this stabilisation time, the pressure
change in the test piece is compared tothe pressure change in the reference
volume, using the Transducer. If the testpiece is leaking, the difference willincrease and be measured, an alarm limit
may be set for a pass/fail decision.
The sequence is fully automatic, the
accuracy and sensitivity of the system isdefined by the method of setting the
preset pressure together with the qualityand type of control valves andDifferential Pressure Transducer.
helium systems
A vacuum pump evacuates the testchamber and test piece simultaneously
to a preset vacuum. At this preset level,the chamber and the test piece areisolated and the chamber evacuated
further to a very low pressure. Apositive pressure variation is therefore
created between the test piece and thechamber.
Helium gas is then introduced into thetest piece, often in a 10% concentration.A Mass Spectrometer analyses a
sample from the chamber as the vacuumcontinues to be drawn. The MassSpectrometer measures the helium
leakage and sets the pass/fail decision.
The test piece pressure is often
compared to the chamber pressurebefore dosing with helium, to avoid
saturating the Mass Spectrometer in the
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